The design of such hand prostheses presents formidable problems in achieving gripping light enough to handle fragile objects and strong enough to provide a secure grip to objects where it is desired to apply substantial forces. These problems are further compounded by the limited spaced available, especially within the fingers themselves, and where it is required to provide for independent movement of different fingers to a greater or lesser extent, and/or varying the rate of movement of individual fingers.
A particular problem arises with the fitting of hand prostheses in patients with relatively long hand stumps. Conventional electrical hand prostheses use an electric motor mounted in the body structure of the hand itself. Some versions have the motor mounted axially parallel to the long axis of the arm and others have the motor at 90 degrees to this axis. A variety of transmission systems are used to link motor and fingers e.g. lead screw and nut, or bevel and spur gears. Without exception these types of hand prosthesis require precise alignment between motor and transmission system. This is usually achieved by locating all the parts within a hand body consisting of an investment casting or other moulded structure. Such an arrangement is relatively cumbersome and the motor cannot be readily accommodated within the limited space available in the main body of the prosthesis attached to the patient's hand stump. Another disadvantage is that patients with some residual digits cannot have any functional restoration at all because of the space constraints. The operational characteristics of such devices also tend to be restricted e.g. allowing only a single gripping pattern to be employed.